Influence of Surface Roughness on the Oxide-Scale Growth of Fe-15Cr Alloy at High Temperature

Abstract:

Article Preview

This present work dealt with the effect of surface roughness on the scales growth of Fe-15Cr alloy. Surface morphologies and oxidation kinetics reveal that there was no obvious influence of surface roughness on the oxide-scale growth of Fe-15Cr alloy except for the initial oxidation stage of 1 h. However, there was an obvious influence on the oxide-scale spallation and microstructure, especially under the mutual function of temperature changes and surface roughness.

Abstract: Small amounts of silver have been added to the intermetallic compound β-NiAl for the
purpose of improving its mechanical properties. Four ternary NiAl-Ag alloys NiAl-0.5Ag, NiAl-1Ag, NiAl-5Ag and NiAl-10Ag (at.%), and an Ag-free β-NiAl have been oxidized at 900oC for 24 h in 1 atm O2 to study the effect of the presence of silver on the oxidation of β-NiAl. The kinetics of all the alloys were generally composed of two main parabolic stages with slightly larger parabolic rate constants for the second stage, except for NiAl-10Ag, which has an instantaneous parabolic rate constant decreasing with time. A continuous external layer of Al2O3 formed on all the alloys. In particular, the scales formed on NiAl-5Ag and NiAl-10Ag contained a thin and discontinuous layer of silver at the alloy/Al2O3 interface. Furthermore, NiAl-10Ag formed also isolated Ag particles or even a discontinuous Ag layer occasionally surmounting the Al2O3 scale. The addition of minor amounts of silver does not affect significantly the oxidation of β-NiAl, because silver is essentially present as a second phase due to its very small solubility in this intermetallic compound.

Abstract: Many high-temperature coatings rely on the formation of a continuous and adherent
thermally grown oxide (TGO) scale of α-Al2O3 for extended resistance to degradation. For instance,
the durability and reliability of thermal barrier coating (TBC) systems in gas turbines are critically
linked to the oxidation behavior and stability of an alumina-forming β-NiAl-based bond coat. This
study focuses primarily on the development of unique Pt+Hf-modified γ′-Ni3Al+γ-Ni coating
compositions that form highly adherent, slow-growing TGO scales during both isothermal and
cyclic oxidation at high temperature. Recent findings on the isothermal and cyclic oxidation
behavior of γ′+γ alloys and coatings will be discussed, with particular emphasis on the effects of Pt,
Al and Hf contents and distributions. Inferred reasons for the observed “Pt effect” will also be
presented.

Abstract: The high temperature oxidation and microstructure evolution of 55%Al-Zn-Si coated sheets were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). After oxidation, the coatings consisted of three phases including ZnO, Fe2Al5, and FeAl from topcoat to the substrate. The different diffusion rate of Fe and Al result in forming voids at the interface of intermetallic layer and the substrate. A good agreement has been reached between the experimental data and the calculation from Chou diffusion model, which has a good predicted function. Moreover, the characteristic oxidation time and the apparent activation energy were obtained.

Abstract: Phosphoric acid treatment is used as a way to improve the high temperature oxidation resistance of a chromia-forming AISI 304 steel. Chromia-forming steels are excellent candidates to resist to high temperature oxidizing atmospheres because of the formation of protective oxide scales. The oxide scale growth mechanisms are studied by exposing phosphoric acid-treated and untreated 304 steel samples to high temperature conditions in air. The analyses were carried out by means of thermogravimetry, and in situ X-ray diffraction (XRD). The experimental results show that the phosphoric acid treatment does not have a beneficial effect on cyclic high temperature oxidation (up to 70h of the oxidation test) of AISI 304 steel because of growth of a layer mainly formed by external cation diffusion which grows very quickly. The isothermal high temperature oxidation of this steel at 800°C in air shows a very fast initial iron oxidation towards the external interface, allowing to chromium element to be more available to the internal interface to form a continuous chromia layer, thus causing the establishment of a parabolic oxidation regime and leading to a beneficial reduction of the oxidation rate (after 70h of the oxidation test).